Grand duke is a European hereditary title, used either by certain monarchs or by members of certain monarchs' families. In status, a grand duke traditionally ranks in order of precedence below an emperor, King, grand prince, archduke, or prince-archbishop, and above a sovereign prince or sovereign duke. The title is used in some current and former independent monarchies in Europe, particularly:in the present-day Grand Duchy of Luxembourg historically by the sovereigns of former independent countries, such as Tuscany in Baden, Hesse, Oldenburg, Mecklenburg-Schwerin, Mecklenburg-Strelitz and Saxe-Weimar – grand duchies from 1815 to 1918, and all now part of present-day Germany formerly also in some countries in Northern Europe, such as the Grand Duchy of Finland or the Grand Duchy of Lithuania

A monarch is a head of state for life or until abdication, and therefore the head of state of a monarchy. A monarch may exercise the highest authority and power in the state, or others may wield that power on behalf of the monarch. Usually a monarch either personally inherits the lawful right to exercise the state's sovereign rights or is selected by an established process from a family or cohort eligible to provide the nation's monarch. Alternatively, an individual may proclaim oneself monarch, which may be backed and legitimated through acclamation, right of conquest or a combination of means.